Expandable stents and method for making same

ABSTRACT

The invention is directed to an expandable stent for implantation in a body lumen, such as an artery, and a method for making it from a single length of tubing. The stent consists of a plurality of radially expandable cylindrical elements generally aligned on a common axis and interconnected by one or more interconnective elements. The individual radially expandable cylindrical elements consist of ribbon-like material disposed in an undulating pattern. Portions of the expanded stent project outwardly into engagement with the vessel wall to more securely attach the stent.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of co-pending U.S.patent application Ser. No. 08/164,986 filed Dec. 9, 1993, which is acontinuation application of U.S. Ser. No. 07/783,558 filed Oct. 28,1991, now abandoned.

BACKGROUND OF THE INVENTION

[0002] This invention relates to expandable endoprosthesis devices,generally called stents, which are adapted to be implanted into apatient's body lumen, such as blood vessel, to maintain the patencythereof. These devices are very useful in the treatment ofatherosclerotic stenosis in blood vessels.

[0003] Stents are generally tubular-shaped devices which function tohold open a segment of a blood vessel or other anatomical lumen. Theyare particularly suitable for use to support and hold back a dissectedarterial lining which can occlude the fluid passageway therethrough.

[0004] Further details of prior art stents can be found in U.S. Pat. No.3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,338 (Balko et al.); U.S.Pat. No. 4,553,545 (Maass et al.); U.S. Pat. No. 4,733,665 (Palmaz);U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882(Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); and U.S. Pat. No.4,886,062 (Wiktor), which are hereby incorporated herein in theirentirety by reference thereto.

[0005] Various means have been described to deliver and implant stents.One method frequently described for delivering a stent to a desiredintraluminal location includes mounting the expandable stent on anexpandable member, such as a balloon, provided on the distal end of anintravascular catheter, advancing the catheter to the desired locationwithin the patient's body lumen, inflating the balloon on the catheterto expand the stent into a permanent expanded condition and thendeflating the balloon and removing the catheter. One of the difficultiesencountered using prior stents involved maintaining the radial rigidityneeded to hold open a body lumen while at the same time maintaining thelongitudinal flexibility of the stent to facilitate its delivery.

[0006] What has been needed and heretofore unavailable is a stent whichhas a high degree of flexibility so that it can be advanced throughtortuous passageways and can be readily expanded and yet have themechanical strength to hold open the body lumen into which it expanded.The present invention satisfies this need.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to an expandable stent which isrelatively flexible along its longitudinal axis to facilitate deliverythrough tortuous body lumens, but which is stiff and stable enoughradially in an expanded condition to maintain the patency of a bodylumen such as an artery when implanted therein.

[0008] The stent of the invention generally includes a plurality ofradially expandable cylindrical elements which are relativelyindependent in their ability to expand and to flex relative to oneanother. The individual radially expandable cylindrical elements of thestent are dimensioned so as to be longitudinally shorter than their owndiameters. Interconnecting elements or struts extending between adjacentcylindrical elements provide increased stability and a preferableposition to prevent warping of the stent upon the expansion thereof. Theresulting stent structure is a series of radially expandable cylindricalelements which are spaced longitudinally close enough so that smalldissections in the wall of a body lumen may be pressed back intoposition against the lumenal wall, but not so close as to compromise thelongitudinal flexibilities of the stent. The individual cylindricalelements may rotate slightly relative to adjacent cylindrical elementswithout significant deformation, cumulatively giving a stent which isflexible along its length and about its longitudinal axis but is stillvery stiff in the radial direction in order to resist collapse.

[0009] The stent embodying features of the invention can be readilydelivered to the desired lumenal location by mounting it on anexpandable member of a delivery catheter, for example a balloon, andpassing the catheter-stent assembly through the body lumen to theimplantation site. A variety of means for securing the stent to theexpandable member on the catheter for delivery to the desired locationare available. It is presently preferred to compress the stent onto theballoon. Other means to secure the stent to the balloon includeproviding ridges or collars on the inflatable member to restrain lateralmovement, or using bioresorbable temporary adhesives.

[0010] The presently preferred structure for the expandable cylindricalelements which form the stents of the present invention generallycircumferential undulating pattern, e.g. serpentine. The transversecross-section of the undulating component of the cylindrical element isrelatively small and preferably has an apect ratio of about two to oneto about 0.5 to one. A one to one apect ratio has been foundparticularly suitable. The open reticulated structure of the stentallows for the perfusion of blood over a large portion of the arterialwall which can improve the healing and repair of a damaged arteriallining.

[0011] The radial expansion of the expandable cylinder deforms theundulating pattern thereof similar to changes in a waveform which resultfrom decreasing the waveform's amplitude and the frequency. Preferably,the undulating patterns of the individual cylindrical structures are inphase with each other in order to prevent the contraction of the stentalong its length when it is expanded. The cylindrical structures of thestent are plastically deformed when expanded (except with NiTi alloys)so that the stent will remain in the expanded condition and thereforethey must be sufficiently rigid when expanded to prevent the collapsethereof in use. During expansion of the stent, portions of theundulating pattern will tip outwardly resulting in projecting members onthe outer surface of the expanded stent. These projecting members tipradially outwardly from the outer surface of the stent and embed in thevessel wall and help secure the expanded stent so that it does not moveonce it is implanted.

[0012] With superelastic NiTi alloys, the expansion occurs when thestress of compression is removed so as to allow the phase transformationfrom austenite back to martensite and as a result the expansion of thestent.

[0013] The elongated elements which interconnect adjacent cylindricalelements should have a transverse cross-section similar to thetransverse dimensions of the undulating components of the expandablecylindrical elements. The interconnecting elements may be formed in aunitary structure with the expandable cylindrical elements from the sameintermediate product, such as a tubular element, or they may be formedindependently and connected by suitable means, such as by welding or bymechanically securing the ends of the interconnecting elements to theends of the expandable cylindrical elements. Preferably, all of theinterconnecting elements of a stent are joined at either the peaks orthe valleys of the undulating structure of the cylindrical elementswhich for the stent. In this manner there is no shortening of the stentupon expansion.

[0014] The number and location of elements interconnecting adjacentcylindrical elements can be varied in order to develop the desiredlongitudinal flexibility in the stent structure both in the unexpandedas well as the expanded condition. These properties are important tominimize alteration of the natural physiology of the body lumen intowhich the stent is implanted and to maintain the compliance of the bodylumen which is internally supported by the stent. Generally, the greaterthe longitudinal flexibility of the stent, the easier and the moresafely it can be delivered to the implantation site.

[0015] In a presently preferred embodiment of the invention the stent isconveniently and easily formed by coating stainless steel tubing with amaterial resistant to chemical etching, removing portions of the coatingto expose portions of underlying tubing which are to be removed todevelop the desired stent structure. The exposed portions of the tubingare removed by chemically etching from the tubing exterior leaving thecoated portion of the tubing material in the desired pattern of thestent structure. The etching process develops smooth openings in thetubing wall without burrs or other artifacts which are characteristic ofmechanical or laser machining processes in the small sized productscontemplated. Moreover, a computer controlled laser patterning processto remove the chemical resistive coating makes photolithographytechnology adaptable to the manufacture of these small products. Theforming of a mask in the extremely small sizes needed to make the smallstents of the invention would be a most difficult task. A plurality ofstents can be formed from one length of tubing by repeating the stentpattern and providing small webs or tabs to interconnect the stents.After the etching process, the stents can be separated by severing thesmall webs or tabs which connect them.

[0016] Other features and advantages of the present invention willbecome more apparent from the following detailed description of theinvention. When taken in conjunction with the accompanying exemplarydrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is an elevational view, partially in section, of a stentembodying features of the invention which is mounted on a deliverycatheter and disposed within a damaged artery.

[0018]FIG. 2 is an elevational view, partially in section, similar tothat shown in FIG. 1 wherein the stent is expanded within a damagedartery, pressing the damaged lining against the arterial wall.

[0019]FIG. 3 is an elevational view, partially in section showing theexpanded stent within the artery after withdrawal of the deliverycatheter.

[0020]FIG. 4 is a perspective view of a stent embodying features of theinvention in an unexpanded state, with one end of the stent being shownin an exploded view illustrate the details thereof.

[0021]FIG. 5 is a plan view of a flattened section of a stent of theinvention which illustrates the undulating pattern of the stent shown inFIG. 4.

[0022]FIG. 6 is a schematic representation of equipment for selectivelyremoving coating applied to tubing in the manufacturing of the stents ofthe present invention.

[0023]FIGS. 7 through 10 are perspective views schematicallyillustrating various configurations of interconnective element placementbetween the radially expandable cylindrical elements of the stent.

[0024]FIG. 11 is a plan view of a flattened section of a stentillustrating an alternate undulating pattern in the expandablecylindrical elements of the stent which are out of phase.

[0025]FIG. 12 is an enlarged partial view of the stent of FIG. 5 withthe various members slightly expanded.

[0026]FIG. 13 is a perspective view of the stent of FIG. 4 after it isfully expanded depicting some members projecting radially outwardly.

[0027]FIG. 14 is an enlarged, partial perspective view of one U-shapedmember with its tip projecting outwardly after expansion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028]FIG. 1 illustrates a stent 10 incorporating features of theinvention which is mounted onto a delivery catheter 11. The stentgenerally comprises a plurality of radially expandable cylindricalelements 12 disposed generally coaxially and interconnected by elements13 disposed between adjacent cylindrical elements. The delivery catheter11 has an expandable portion or balloon 14 for expanding of the stent 10within an artery 15. The artery 15, as shown in FIG. 1 has a dissectedlining 16 which has occluded a portion of the arterial passageway.

[0029] The delivery catheter 11 onto which the stent 10 is mounted, isessentially the same as a conventional balloon dilatation catheter forangioplasty procedures. The balloon 14 may be formed of suitablematerials such as polyethylene, polyethylene terephthalate, polyvinylchloride, nylon and ionomers such as Surlyn® manufactured by the PolymerProducts Division of the Du Pont Company. Other polymers may also beused. In order for the stent 10 to remain in place on the balloon 14during delivery to the site of the damage within the artery 15, thestent 10 is compressed onto the balloon. A retractable protectivedelivery sleeve 20 as described in co-pending applications Ser. No.07/647,464 filed on Apr. 25, 1990 and entitled STENT DELIVERY SYSTEM maybe provided to further ensure that the stent stays in place on theexpandable portion of the delivery catheter 11 and prevent abrasion ofthe body lumen by the open surface of the stent 20 during delivery tothe desired arterial location. Other means for securing the stent 10onto the balloon 14 may also be used, such as providing collars orridges on the ends of the working portion, i.e. the cylindrical portion,of the balloon.

[0030] Each radially expandable cylindrical element 12 of the stent 10may be independently expanded. Therefore, the balloon 14 may be providedwith an inflated shape other than cylindrical, e.g. tapered, tofacilitate implantation of the stent 10 in a variety of body lumenshapes.

[0031] In a preferred embodiment, the delivery of the stent 10 isaccomplished in the following manner. The stent 10 is first mounted ontothe inflatable balloon 14 on the distal extremity of the deliverycatheter 11. The balloon 14 is slightly inflated to secure the stent 10onto the exterior of the balloon. The catheter-stent assembly isintroduced within the patient's vasculature in a conventional Seldingertechnique through a guiding catheter (not shown). A guidewire 18 isdisposed across the damaged arterial section with the detached ordissected lining 16 and then the catheter-stent assembly is advancedover a guidewire 18 within the artery 15 until the stent 10 is directlyunder the detached lining 16. The balloon 14 of the catheter isexpanded, expanding the stent 10 against the artery 15, which isillustrated in FIG. 2. While not shown in the drawing, the artery 15 ispreferably expanded slightly by the expansion of the stent 10 to seat orotherwise fix the stent 10 to prevent movement. In some circumstancesduring the treatment of stenotic portions of an artery, the artery mayhave to be expanded considerably in order to facilitate passage of bloodor other fluid therethrough.

[0032] The stent 10 serves to hold open the artery 15 after the catheter11 is withdrawn, as illustrated by FIG. 3. Due to the formation of thestent 10 from elongated tubular member, the undulating component of thecylindrical elements of the stent 10 is relatively flat in transversecross-section, so that when the stent is expanded, the cylindricalelements are pressed into the wall of the artery 15 and as a result donot interfere with the blood flow through the artery 15. The cylindricalelements 12 of stent 10 which are pressed into the wall of the artery 15will eventually be covered with endothelial cell growth which furtherminimizes blood flow interference. The undulating portion of thecylindrical sections 12 provide good tacking characteristics to preventstent movement within the artery. Furthermore, the closely spacedcylindrical elements 12 at regular intervals provide uniform support forthe wall of the artery 15, and consequently are well adapted to tack upand hold in place small flaps or dissections in the wall of the artery15 as illustrated in FIGS. 2 and 3.

[0033]FIG. 4 is an enlarged perspective view of the stent 10 shown inFIG. 1 with one end of the stent shown in an exploded view to illustratein greater detail the placement of interconnecting elements 13 betweenadjacent radially expandable cylindrical elements 12. Each pair of theinterconnecting elements 13 on one side of a cylindrical element 12 arepreferably placed to achieve maximum flexibility for a stent. In theembodiment shown in FIG. 4 the stent 10 has three interconnectingelements 13 between adjacent radially expandable cylindrical elements 12which are 120 degrees apart. Each pair of interconnecting elements 13 onone side of a cylindrical element 12 are offset radially 60 degrees fromthe pair on the other side of the cylindrical element. The alternationof the interconnecting elements results in a stent which islongitudinally flexible in essentially all directions. Variousconfigurations for the placement of interconnecting elements arepossible, and several examples are illustrated schematically in FIGS.7-10. However, as previously mentioned, all of the interconnectingelements of an individual stent should be secured to either the peaks orvalleys of the undulating structural elements in order to preventshortening of the stent during the expansion thereof.

[0034]FIG. 10 illustrates a stent of the present invention wherein threeinterconnecting elements 12 are disposed between radially expandablecylindrical elements 11. The interconnecting elements 12 are distributedradially around the circumference of the stent at a 120-degree spacing.Disposing four or more interconnecting elements 13 between adjacentcylindrical elements 12 will generally give rise to the sameconsiderations discussed above for two and three interconnectingelements.

[0035] The properties of the stent 10 may also be varied by alterationof the undulating pattern of the cylindrical elements 13. FIG. 11illustrates an alternative stent structure in which the cylindricalelements are in serpentine patterns but out of phase with adjacentcylindrical elements. The particular pattern and how many undulationsper unit of length around the circumference of the cylindrical element13, or the amplitude of the undulations, are chosen to fill particularmechanical requirements for the stent such as radial stiffness.

[0036] The number of undulations may also be varied to accommodateplacement of interconnecting elements 13, e.g. at the peaks of theundulations or along the sides of the undulations as shown in FIGS. 5and 11.

[0037] In keeping with the invention, and with reference to FIGS. 4 and12-14, cylindrical elements 12 are in the form of a serpentine pattern30. As previously mentioned, each cylindrical element 12 is connected byinterconnecting elements 13. Serpentine pattern 30 is made up of aplurality of U-shaped members 31, W-shaped members 32, and Y-shapedmembers 33, each having a different radius so that expansion forces aremore evenly distributed over the various members.

[0038] As depicted in FIGS. 13 and 14, after cylindrical elements 12have been radially expanded, outwardly projecting edges 34 are formed.That is, during radial expansion U-shaped members 31 will tip outwardlythereby forming outwardly projecting edges. These outwardly projectingedges provide for a roughened outer wall surface of stent 10 and assistin implanting the stent in the vascular wall by embedding into thevascular wall. In other words, outwardly projecting edges embed into thevascular wall, for example artery 15, as depicted in FIG. 3. Dependingupon the dimensions of stent 10 and the thickness of the various membersmaking up the serpentine pattern 30, any of the U-shaped members 31,W-shaped members 32, and Y-shaped members 33 can tip radially outwardlyto form a projecting edge 34. It is most likely and preferred thatU-shaped members 31 tip outwardly since they do not join with anyconnecting member 13 to prevent them from expanding outwardly.

[0039] The stent 10 of the present invention can be made in many ways.However, the preferred method of making the stent is to coat athin-walled tubular member, such as stainless steel tubing, with amaterial which is resistive to chemical etchants, remove portions of thecoating to expose underlying tubing which is to be removed but to leavecoated portions of the tubing in the desired pattern for the stent sothat subsequent etching will remove the exposed portions of the metallictubing, but will leave relatively untouched the portions of the metallictubing which are to form the stent. The coated portion of the metallictube is in the desired shape for the stent. An etching process avoidsthe necessity of removing burrs or slag inherent in conventional orlaser machining process. It is preferred to remove the etchant-resistivematerial by means of a machine-controlled laser as illustratedschematically in FIG. 6.

[0040] A coating is applied to a length of tubing which, when cured, isresistive to chemical etchants. “Blue Photoresist” made by the ShipleyCompany in San Jose, Calif., is an example of suitable commerciallyavailable photolithographic coatings. The coating is preferably appliedby electrophoretic deposition.

[0041] To ensure that the surface finish is reasonably uniform, one ofthe electrodes used for the electrochemical polishing is adoughnut-shaped electrode which is placed about the central portion ofthe tubular member.

[0042] The tubing may be made of suitable biocompatible material such asstainless steel, titanium, tantalum, superelastic NiTi alloys and evenhigh strength thermoplastic polymers. The stent diameter is very small,so the tubing from which it is made must necessarily also have a smalldiameter. Typically the stent has an outer diameter on the order ofabout 0.06 inch in the unexpanded condition, the same outer diameter ofthe tubing from which it is made, and can be expanded to an outerdiameter of 0.1 inch or more. The wall thickness of the tubing is about0.003 inch. In the instance when the stent was plastic, it would have tobe heated within the arterial site where the stent is expanded tofacilitate the expansion of the stent. Once expanded, it would then becooled to retain its expanded state. The stent may be convenientlyheated by heating the fluid within the balloon or the balloon directlyby a suitable system such as disclosed in a co-pending application Ser.No. 07/521,337, filed Jan. 26, 1990 entitled DILATATION CATHETERASSEMBLY WITH HEATED BALLOON which is incorporated herein in itsentirety by reference. The stent may also be made of materials such assuperelastic NiTi alloys such as described in co-pending applicationSer. No. 07/629,381, filed Dec. 18, 1990, entitled SUPERELASTIC GUIDINGMEMBER which is incorporated herein in its entirety by reference. Inthis case the stent would be formed full size but deformed (e.g.compressed) into a smaller diameter onto the balloon of the deliverycatheter to facilitate transfer to a desired intraluminal site. Thestress induced by the deformation transforms the stent from a martensitephase to an austenite phase and upon release of the force, when thestent reaches the desired intraluminal location, allows the stent toexpand due to the transformation back to the martensite phase.

[0043] Referring to FIG. 6, the coated tubing 21 is put in a rotatablecollet fixture 22 of a machine controlled apparatus 23 for positioningthe tubing 21 relative to a laser 24. According to machine-encodedinstructions, the tubing 21 is rotated and moved longitudinally relativeto the laser 24 which is also machine controlled. The laser selectivelyremoves the etchant-resistive coating on the tubing by ablation and apattern is formed such that the surface of the tube that is to beremoved by a subsequent chemical etching process is exposed. The surfaceof the tube is therefore left coated in the discrete pattern of thefinished stent.

[0044] A presently preferred system for removing the coating on thetubing includes the use an 80-watt CO₂ laser, such as a Coherent Model44, in pulse mode (0.3 mS pulse length); 48 mA key current and 48 W keypower with 0.75 W average power, at 100 Hz; Anorad FR=20; 12.5 Torr;with no assist gas. Low pressure air is directed through the fine focushead to ensure that no vapor contacts the lens. The assist gas jetassembly on the laser unit may be removed to allow a closer proximity ofthe fine focus head and the collet fixture. Optimum focus is set at thesurface of the tubing. Cured photo-resist coating readily absorbs theenergy of the CO₂ wavelength, so that it can be readily removed by thelaser. A coated 4-inch length of 0.06 inch stainless steel tubing ispreferred and four stents can be patterned on the length of tubing.Three tabs or webs between stents provide good handling characteristicsfor the tubing after the etching process.

[0045] The process of patterning the resistive coating on the stent isautomated except for loading and unloading the length of tubing.Referring again to FIG. 6 it may be done, for example, using aCNC-opposing collet fixture 22 for axial rotation of the length oftubing, in conjunction with a CNC X/Y table 25 to move the length oftubing axially relative to a machine-controlled laser as described. Theentire space between collets can be patterned using the CO₂ laser set-upof the foregoing example. The program for control of the apparatus isdependent on the particular configuration used and the pattern to beablated in the coating, but is otherwise conventional.

[0046] This process makes possible the application of presentphotolithography technology in manufacturing the stents. While there ispresently no practical way to mask and expose a tubular photo-resistcoated part of the small size required for making intravascular stents,the foregoing steps eliminate the need for conventional maskingtechniques.

[0047] After the coating is thus selectively ablated, the tubing isremoved from the collet fixture 22. Next, wax such at ThermoCote N-4 isheated to preferably just above its melting point, and inserted into thetubing under vacuum or pressure. After the wax has solidified uponcooling, it is reheated below its melting point to allow softening, anda smaller diameter stainless steel shaft is inserted into the softenedwax to provide support. The tubing is then etched chemically in aconventional manner. After cutting the tabs connecting the stents anysurface roughness or debris from the tabs is removed. The stents arepreferably electrochemically polished in an acidic aqueous solution suchas a solution of ELECTRO-GLO #300, sold by the ELECTRO-GLO CO., Inc. inChicago, Ill., which is a mixture of sulfuric acid, carboxylic acids,phosphates, corrosion inhibitors and a biodegradable surface activeagent. The bath temperature is maintained at about 110-135 degrees F.and the current density is about 0.4 to about 1.5 amps per in.² Cathodeto anode area should be at least about two to one. The stents may befurther treated if desired, for example by applying a biocompatiblecoating.

[0048] While the invention has been illustrated and described herein interms of its use as an intravascular stent, it will be apparent to thoseskilled in the art that the stent can be used in other instances such asto expand prostatic urethras in cases of prostate hyperplasia. Othermodifications and improvements may be made without departing from thescope of the invention.

[0049] Other modifications and improvements can be made to the inventionwithout departing from the scope thereof.

What is claimed is:
 1. A longitudinally flexible stent for implanting ina body lumen, comprising: a plurality of cylindrical elements which areindependently expandable in the radial direction and which areinterconnected so as to be generally aligned on a common longitudinalaxis; a plurality of connecting elements for interconnecting saidcylindrical elements, said connecting elements configured tointerconnect only said cylindrical elements that are adjacent to eachother; and an outer wall surface on said cylindrical elements, saidouter wall surface having a plurality of outwardly projecting edgeswhich form as said stent is expanded radially outwardly from a firstdiameter to a second, enlarged diameter.
 2. The stent of claim 1,wherein said outer wall surface is substantially smooth when said stentin said first diameter configuration and said outwardly projecting edgesform only as said stent is expanded radially outwardly from said firstdiameter to said second, enlarged diameter.
 3. The stent of claim 1,wherein said plurality of outwardly projecting edges extend a distancefrom said outer wall surface sufficient enough to embed in the vascularwall of the body lumen in order to more firmly attach said stent to thevascular wall.
 4. The stent of claim 1, wherein said plurality ofcylindrical elements include a plurality of peaks and valleys having aserpentine pattern.
 5. The stent of claim 4, wherein said plurality ofpeaks and valleys include a plurality of U-shaped members, a pluralityof Y-shaped members, and a plurality of W-shaped members, some of saidU-shaped, Y-shaped, and W-shaped members being interconnected.
 6. Thestent of claim 5, wherein at least some of said plurality of saidU-shaped members tip radially outwardly to form said outwardlyprojecting edges upon radial expansion of said stent.
 7. The stent ofclaim 5, wherein at least some of said plurality of U-shaped, W-shaped,and Y-shaped members tip radially outwardly to form said outwardlyprojecting edges upon radial expansion of said stent.
 8. The stent ofclaim 1, wherein said cylindrical elements are capable of retainingtheir expanded condition upon the expansion thereof.
 9. The stent ofclaim 1, wherein said stent is formed of a biocompatible materialselected from the group of materials consisting of stainless steel,tantalum, NiTi alloys, and thermoplastic polymers.
 10. The stent ofclaim 1, wherein said stent is formed from a single piece of tubing. 11.The stent of claim 1, wherein said stent is coated with a biocompatiblecoating.
 12. A longitudinally flexible stent, comprising: a plurality ofcylindrical elements which are independently expandable in the radialdirection and which are interconnected so as to be concentricallyaligned on a common longitudinal axis; and a plurality of generallyparallel connecting elements for interconnecting said cylindricalelements, said connecting elements configured to interconnect only saidcylindrical elements that are adjacent to each other, so that saidstent, when expanded radially outwardly, retains its overall lengthwithout appreciable shortening.
 13. The stent of claim 12, wherein saidcylindrical elements are capable of retaining their expanded conditionupon the expansion thereof.
 14. The stent of claim 12, wherein saidradially expandable cylindrical elements in an expanded condition have alength less than the diameter thereof.
 15. The stent of claim 14,wherein said stent is formed of a biocompatible material selected fromthe group consisting of stainless steel, tantalum, super-elastic NiTialloys, and thermoplastic polymers.
 16. The stent of claim 12, whereinsaid connecting elements between adjacent cylindrical elements are inaxial alignment.
 17. The stent of claim 12, wherein said connectingelements between adjacent cylindrical elements are circumferentiallydisplaced with respect to said longitudinal axis.
 18. The stent of claim17, wherein the circumferential displacement of said connecting elementsbetween adjacent cylindrical elements is uniform.
 19. The stent of claim12, wherein there are up to four of said connecting elements disposedbetween adjacent radially expandable cylindrical elements.
 20. The stentof claim 12, wherein said radially expandable cylindrical elements andsaid connecting elements are made of the same material.
 21. The stent ofclaim 12, wherein said stent is formed from a single piece of tubing.22. The stent of claim 12, wherein the stent is coated with abiocompatible coating.
 23. A kit of parts, comprising: an elongatedstent delivery catheter having a proximal end and a distal end, and anexpandable member on the distal end; and a longitudinally flexible stentwhich is adapted to be slidably mounted onto the expandable member ofsaid catheter and which comprises a plurality of cylindrical elementswhich are independently expandable in the radial direction and which areinterconnected so as to be concentrically aligned on a commonlongitudinal axis, wherein each said element is formed of a singleelongated structural member forming a serpentine pattern havingundulations with peaks and valleys, said elements being interconnectedby a plurality of generally parallel interconnecting members betweenadjacent elements, each said interconnecting member configured tointerconnect only said cylindrical elements that are adjacent to eachother.
 24. A method of transluminally implanting a longitudinallyflexible stent in a body lumen, said stent having a plurality ofcylindrical elements which are independently expandable in the radialdirection and which are interconnected so as to be concentricallyaligned on a common longitudinal axis, wherein each said cylindricalelement is interconnected a plurality of generally parallel connectingmembers between adjacent elements, the method comprising the steps of:placing the stent on an expandable portion of a catheter which isadapted to radially expand the stent; delivering the stent to a desiredlocation within the body lumen; expanding said cylindrical elements withthe expandable portion of the catheter; contracting the expandableportion of the catheter; and withdrawing the catheter, leaving theexpanded stent implanted in the body lumen.